Substitution of threonine for glutamine in oxytocin and mesotocin brought about a selected enhancement of oxytocin-like characteristics while diminishing the vasopressin-like properties. The same substitution in the vasopressins had similar effects, but in addition, the antidiuretic to pressor ratio was enhanced two-three fold in the 4- threonine-containing peptides as compared to the 4-glutamine-containing ones. Subsequently, examination of the deamino-derivatives of the 4- threonine substituted vasopressins showed that the antidiuretic to pressor selectivity was increased to 25 and 79 for the 8-Arg and 8-Lys derivatives respectively. These data, when looked at in conjunction with the findings from other laboratories point, in the case of the oxytocin analogs, to the importance of a delicate lipophilic/hydrophilic balance in the 1 and 4 positions for the manifestation of selected oxytocin-like properties and in the case of the vasopressin analogs to the importance of lipophilicity at positions 1 and 4 in leading to profound enhancements of antidiuretic/pressor selectivity. This information provides a useful basis for the design of oxytocin and vasopressin analogs possessing potential clinical value and has led to the synthesis of 1-deamino-4-valine-8-D-arginine vasopressin (dVDAVP). dVDAVP is the most highly specific antidiuretic agent known to date. It also has a very significant protracted effect. These properties make it potentially useful for the treatment of diabetes insipidus.